A gifted computer scientist, Mayer suspected that online advertisers might be getting around browser settings that are designed to block tracking devices known as cookies. If his instinct was right, advertisers were following people as they moved from one website to another even though their browsers were configured to prevent this sort of digital shadowing. Working long hours at his office, Mayer ran a series of clever tests in which he purchased ads that acted as sniffers for the sort of unauthorized cookies he was looking for. He hit the jackpot, unearthing one of the biggest privacy scandals of the past year: Google was secretly planting cookies on a vast number of iPhone browsers. Mayer thinks millions of iPhones were targeted by Google.